1. Field of the Invention
This invention relates to a resonator-type surface acoustic wave (SAW) filter using a surface acoustic wave, and particularly, relates to a transversally coupled double-mode SAW filter in which two SAW resonators are disposed transversally and in parallel.
2. Description of Related Art
Recently, SAW filters, especially transversally coupled multi-mode resonator-type SAW filters, have been widely used as the intermediate-frequency (IF) filters of mobile communication terminals such as portable telephone because they are small, low-loss, have a narrow-band passband characteristics and also less out-of-band unwanted modes. For example, in the transversally coupled double-mode SAW filter described in Japanese unexamined patent application No. S59-131213, two SAW resonators are disposed in parallel on a piezoelectric substrate in a direction perpendicular to the propagation direction of SAW which is excited by curtain-like electrodes of the SAW resonators, i.e., inter-digital transducer (IDT) electrodes, two independent inherent vibration modes originating in the acoustic coupling of SAW, i.e., a fundamental wave symmetric mode S0 and a fundamental wave anti-symmetric mode A0 are used, and the passband width is determined by the difference between their resonance frequencies f1, f2. Particularly, when a quartz crystal substrate with excellent frequency-temperature characteristics is used, a two-stage cascade connection filter is obtained which is in a planar element the size of 2 mmxc3x976.5 mm and has excellent characteristics of ca. 700 ppm in specific bandwidth and 5 dB in insertion loss.
More recently, digital communication modes such as GSM mode and PHS mode seem to be adopted in a portable telephone with the digitalization of communication systems, thus an even smaller IF filter with a relatively broad passband width of 1,000 ppm has been required. In aforesaid conventional two transversally coupled double-mode SAW filters, however, the resonance frequency difference of the fundamental wave symmetric mode S0 and the fundamental wave anti-symmetric mode A0 could not be more than 700 ppm at a width dimension of 7 to 9xcex (xcex: the wavelength of SAW) of one SAW resonator which gives a filter impedance Z0 of 600-800 xcexa9 considered appropriate before. In order to expand the passband width, the electrode finger cross width of IDT and the gap between SAW resonators must be narrowed, therefore such problems as an increase of insertion loss and deterioration of the degree of suppression of out-of-band frequency characteristics arise.
Moreover, if the IF filter is to be miniaturized, the sum M+N of the number of pairs of electrode fingers of IDT forming these SAW filters (M) and the number of conductors of one-side reflectors (N) needs to be decreased. For example, when they are housed in a container of 3.8xc3x973.8 mm or less in plane size required, more recently, the size of elements is made to be 2xc3x973 mm, therefore the sum M+N of the number of pairs of electrode fingers of the SAW filters (M) and the number of conductors of one-side reflectors (N) must be made to be about 200 or less. It lowers the Q value of the SAW resonators (the resonance sharpness), decreases the excitation intensity of resonance amplitude and displacement transmission coefficient and deteriorates the transmission characteristics of the SAW filters.
Furthermore, the thickness of electrode films should be thickened to remedy a deficiency in the number of conductors of the reflectors and improve the insertion loss, but it increases the spurious level of longitudinal and transversal higher modes and deteriorates the degree of suppression of the out-of-band frequency characteristics.
In Japanese unexamined patent publications No. H4-373304 and No. H9-93079, a transversally coupled triple-mode SAW filter in which three SAW resonators are disposed in a direction perpendicular to the propagation direction of SAW has been proposed to expand the passband without narrowing the electrode finger cross width of IDT. In this SAW filter, however, there is the problem that differences unavoidable in design between filter impedances of adjacent 1st and 2nd SAW resonators and the filter impedances of adjacent 2nd and 3rd SAW resonators occur, respectively, making it difficult to obtain practically good filter characteristics because this generates a ripple in the passband. Still more, in Clemens C. W. Ruppel et al.""s paper xe2x80x9cSAW Devices for Mobile Communication Applicationsxe2x80x9d (The 26th Symposium of Electric Society and Electronic Circuit Technical Committee, page 129-130 (1997)), a transversally coupled multi-mode SAW filter which enables an increase in the frequency difference between two modes, i.e., S0 (fundamental wave symmetric)xe2x80x94S1 (primary symmetric) modes and A0 (fundamental wave anti-symmetric)xe2x80x94A1 (primary anti-symmetric) modes by using these modes and consequently expanding the passband width has been disclosed. However, this SAW filter practically has a dispersion in design and in manufacture. It is feared that this is the reason why the A0 mode is excited to generate the ripple in the S0-S1 mode and why the S1 mode is excited to generate the ripple in the A0-A1 mode. There is also concern that the construction of electrode patterns including the disposition of input/output terminals is complicated, therefore the size of filter cannot be fully miniaturized.
Accordingly, this invention was made to overcome aforethe previous problems, and its purpose consists in providing a transversally coupled multi-mode SAW filter which enables to seek the broadening of passband width and the miniaturization.
Moreover, the purpose of this invention consists of providing a transversally coupled multi-mode SAW filter which has excellent frequency stability and good S/N ratio, and is suited to the use as an IF filter of mobile communication terminals such as portable telephone.
The inventors of this application discovered that the vibration displacement of a so-called transversal mode and the resonance frequency thereof can be calculated by use of a theory described below. This transversal mode is an inherent vibration mode which exists depending on the length of SAW resonators in the cross direction (the Y-axis direction perpendicular to the propagation direction X axis of SAW), and the length in the cross direction generally means the electrode finger cross width WC of IDT, i.e., the dimension of a portion where electrode fingers of positive polarity and electrode fingers of negative polarity overlap each other in the cross direction.
As a method for simply calculating the vibrational displacement of SAW resonators in the cross direction, the inventors of this application have published a differential equation dominating the transversal modes in a paper xe2x80x9cK-cut quartz crystal SAW resonators having dynamic and static zero-temperature coefficients at normal temperaturexe2x80x9d (Takagi, Momosaki et al., the 25th EM Symposium of Electric Society and Electronic Circuit Technical Committee, page 77-83 (1996)). This equation is described as the following Eq. (1).
axcfx8902(Y)V(Y),YY+{xcfx8902(Y)}V(Y)=0xe2x80x83xe2x80x83(1)
Here, xcfx89 is the angular frequency, xcfx890(Y) is the angular frequency of element in the region, a is the effective shear rigidity constant in the cross direction, V(Y) is the amplitude of displacement of surface acoustic wave in the cross direction, and Y is the Y-coordinate specified by the wavelength of the surface acoustic wave. Moreover, xcfx890(Y) is a quantity given by converting the speed of the surface acoustic wave at the coordinate Y to the angular frequency, and is called the frequency potential function.
It has been confirmed that this frequency potential function changes with a function of the thickness H(Y) of aluminum metal conductor film existing in the propagation path of a surface acoustic wave in the vicinity of operating points of SAW resonators and, more generally, changes with a function of the mass m (Y) of aluminum metal. The xcfx890(Y) at the IDT forming the principal part of the SAW resonators is roughly determined by the mass m(Y) of IDT. Namely, the speed of the surface acoustic wave at a coordinate Y can be described as xcfx890(m(Y)). When the employed piezoelectric substrate is quartz crystal ST-cut, the film thickness of IDT is thin, therefore the xcfx890(Y) linearly falls in a rough proportion to m.
The Eq. (1) is a Helmhortz equation when the wavenumber is a function of position, if it is divided by a frequency xcfx89002 based on the whole equation to simplify the calculation, then the equation can be written as the following Eq. (2).
aQ2(Y)V(Y),YY+{xcexa92xe2x88x92Q2(Y)}V(Y)=0xe2x80x83xe2x80x83(2)
Here, xcexa9Q=xcfx89/xcfx8900 is the specified frequency, and Q(m(Y))=xcfx890(Y)/xcfx8900 is the potential function.
The vibrational displacement amplitude V(Y) can be calculated, e.g., by successive integration as follows.                               V          ⁡                      (                          Y              ,              Ω                        )                          =                                            ∫              0              Y                        ⁢                                                            V                  ⁡                                      (                    Y                    )                                                                    ,                  Y                                            ⁢                              ⅆ                Y                                              +                      C            ⁢                          xe2x80x83                        ⁢                          (              constant              )                                                          (        3        )            
however,             (              V        ⁢                  (                      Y            ,            Ω                    )                    )              ,      Y        =      -                  ∫        0        Y            ⁢                        {                                    Ω              2                        -                                          Q                2                            ⁢                              (                Y                )                                              }                ⁢                              V            ⁢                          (              Y              )                                /          a                ⁢                  xe2x80x83                ⁢                              Q            2                    ⁢                      (            Y            )                          ⁢                  ⅆ          Y                    
The V(Y, xcexa9) of Eq. (3) is a function of the specified frequency xcexa9, but the actual vibrational displacement amplitude is realized at xcexa9 given by the following Eq. (4) being the stationary principle of energies because the total integral quantity with respect to Y of V2(Y, xcexa9) being the square of the amplitude is the total energy of the resonator.                                           ∂                          (                              2                ⁢                                  E                  ⁡                                      (                    Ω                    )                                                              )                                /                      ∂            Ω                          =                                            ∂                              (                                                      ∫                    0                    ∞                                    ⁢                                                                                    V                        2                                            ⁡                                              (                                                  Y                          ,                          Ω                                                )                                                              ⁢                                          ⅆ                      Y                                                                      )                                      /                          ∂              Ω                                =          0                                    (        4        )            
Because the resonance frequency of the fundamental wave and higher modes of a transversally coupled double-mode SAW filter can be accurately calculated by using design conditions such as dimensions, film thickness of electrode pattern of the SAW filter based on the above theory, the inventors of this application thought of the use of S1xc2x7A1 modes which was previously considered to be unusable in common sense because a resonance of S0 mode and A0 mode of the strongest fundamental wave generated as spuroius modes, thus they devised this invention.
This invention provides a transversally coupled double-mode SAW filter and
Two SAW resonators, which have curtain-like electrodes and a pair of reflectors on both sides thereof, respectively, are disposed in parallel on a piezoelectric flat plate in the propagation direction of a surface acoustic wave.
The curtain-like electrodes of the SAW resonators have a first bus bar of electrically positive electrode which extends in the propagation direction and second and third bus bars of electrically negative electrode which extend in parallel to both sides of the first bus bar.
The first bus bar has multiple electrode fingers of electrically positive electrode which face to the second and third bus bars on both sides thereof, and which extend in a direction perpendicular to the propagation direction.
The second and third bus bars have multiple electrode fingers of electrically positive electrode which correspond to the electrode fingers of electrically positive electrode, which face to the first bus bar and which extend in a direction perpendicular to the propagation direction.
Thereby, the curtain-like electrodes are constituted to invert the direction of driving electric field by 180xc2x0 with the first bus bar therebetween, and the primary symmetric mode S1 and primary anti-symmetric mode A1 can be excited.
By such a construction, the transmission characteristics of the transversally coupled double-mode resonator type SAW filter are not a fundamental wave but are synthesized from the primary symmetric mode S1 and primary anti-symmetric mode A1 which belong to higher transversal modes, thus a frequence difference between S1xc2x7A1 modes, i.e., passband width sharply broader than before can be obtained. Moreover, the generated charges of S0xc2x7A0 modes are cancelled to suppress the excitation of these modes, and the generated charge of S2 mode is offset to suppress the excitation of this mode, thus the spurious level of S2 mode which was difficult to be suppressed before is as low as 40 dB or below, therefore no ripple occurs in the passband, and this filter is especially suitable as an IF filter of mobile communication terminals with relatively large frequency width between channels of PHS, GSM mode, etc. in practice.
Moreover, because the IDT consisting of 180xc2x0 inverted electrodes for driving the S1xc2x7A1 modes in a good efficiency takes a large electrode finger cross width of up to 8xcex to 12xcex and integrates the generated charges of S1xc2x7A1 modes in a good efficiency without offset, as a result, it enables a decrease in the equivalent resistance of the SAW resonators and improves the insertion loss. Furthermore, such an electrode pattern takes a periodic sequence structure in the propagation direction of SAW, no unreasonableness of wiring in design and does not harm the Q value of the resonators, therefore the miniaturization of elements is easy.
Particularly, the electrode fingers of electrically positive and negative electrodes of the SAW resonators are preferably formed and connected so as to have a periodic space of xcex/2 (xcex: the wavelength of a surface acoustic wave) and a width of xcex/4 for the bus bars.
In one embodiment, the excitation of SAW caused by common noises (voltage) between input/output terminals can be prevented and a good use condition for the noises can be realized by forming the input-side curtain-like electrodes of the SAW resonators into a differential type having input terminals on the anode side and the cathod side which are electrically separately from the ground.
In another embodiment, an electrostatic shielding effect for cutting-off electrostatic coupling noises, i.e., a voltage transmitted from the input to the output via a floating electrostatic capacitance is obtained by forming a one-side grounding input terminal pair consisting of an anode side input terminal electrically separated from the ground and a cathode side input terminal connected to the ground on the output-side curtain-like electrode of the SAW resonators, thus this is convenient.
To secure a bandwidth of 1,000 ppm, it is more preferable that the curtain-like electrodes of the two SAW resonators approaching each other are separated by a gap length E of 2.5to 5 xcexcm, and/or the finger cross width of electrode fingers of electrically positive and negative electrodes of the curtain-like electrodes is set within a range of 4xcex to 6xcex.
More specifically, the planar area of elements can be reduced to about a half as compared to before, small good SAW filters can be realized and can correspond to the miniaturization of mobile communication terminals such as portable telephone by designing the number of pairs of curtain-like electrodes of the SAW resonators in a range as small as 120 to 60 and the number of conductors of one-side reflectors in a range as small as 80 to 140.
Moreover, if the piezoelectric flat plate is a quartz crystal substrate and is a ST-cut X transmission orientation of 30-45xc2x0 rotated Y plate, the frequency-temperature characteristics are highly accurate, the frequency changes in the use temperature range are small and are stabilized, thus this is convenient.
Furthermore, in one embodiment, the out-of-band attenuation can be secured by two-stage cascade connection of aforesaid transversally coupled double-mode SAW filter, thus this is convenient.